High-modulus, high-strength inorganic oxide fibers are used in making many high-performance polymer composite materials. There are generally two types of polymer composites: short fiber reinforced composites and continuous fiber reinforced composites. Short fiber reinforced composites often include thermoplastics and are processed through extrusion processes. Continuous fiber reinforced composites often include thermoset resins wherein the fiber is usually pre-laid and impregnated with the resin to make a so-called “prepreg”, which is then laid and cured to a specific shape. Such composite materials are usually made by extrusion compounding process, where precut short fibers are added into an extruder through a feed port. To facilitate the fiber addition, a fiber sizing agent (i.e., a fiber size) is used to keep the cut fiber in bundles until it is dispersed inside the extruder. Inadequately bundled fibers can hamper the compounding process; for example, by blocking the extruder feed port. Another important role of fiber sizing is to provide high interfacial adhesion between the inorganic oxide fiber and the polymeric matrix. Good fiber-polymeric matrix adhesion provides efficient load transfer between the fiber and the polymeric matrix allowing the inorganic oxide fiber to reinforce the matrix polymer effectively. Selection of appropriate fiber sizes has typically depended on the specific combination of fiber and polymeric matrix.